We plan to determine the relative quantities of (1) ingested protein, (2) pancreatic protein and (3) other endogenous protein in the gut lumen of dogs and rats during digestion. Animals injected with Se75-MET, to label digestive enzymes, will be used as donors to deliver their pancreatic juice to the duodenum of recipient animals, which will deliver their own pancreatic juice to the outside. When the recipient animals are fed H3 or C14-labeled dietary protein the total N and radioactivity of their gut contents will be determined. From these data it will be possible to compute how much of the total N is contributed by exogenous and endogenous protein and how much of the endogenous protein is contributed by the pancreas. Eventually two animals will deliver their pancreatic juice, with different labels to each other while ingesting labeled dietary protein.